Wave Steepness Algorithm

First, we compute a wave steepness parameter, ,
at all frequencies, f:

where Hs is the significant wave height,
L is the wave length associated with Tz and
Tz is the average wave period that are computed from the
nthmoment of the wave spectrum by

where fu and f1 are the upper and lower
frequency limits of measured wave spectra.

The separation frequency fs is estimated by

where fx is the frequency of maximum (f),
and C=0.75, is an empirically determined constant.

Under light winds with swell present, fs often overestimates the
contribution of wind seas. A modification to this method imposes a minimum separation
frequency based on wind speed. Minimum separation frequency is determined from the peak
frequency of the Pierson-Moskowitz spectrum of a fully-developed sea at the observed wind speed by

where fsu is separation frequency and 1.25/U is the peak frequency
of the Pierson-Moskowitz spectrum at the locally observed wind speed adjusted to 10m1,
U10, in m/s. C is an empirically determined constant equal to 0.90.

Note 1. Wind speeds on the NDBC web site are those actually observed by the anemometer
(typically at 5 m) and have not been adjusted. If you have access to NDBC observations issued
by the National Weather Service in WMO CMAN and SHIP code (FM12, FM13 format) wind
speeds adjusted to 10 and 20 meters can be found in section 5, field identifiers 11 and 22.

The method used by NDBC to adjust wind speeds to the standard reference levels is described in
a 1979 paper by W. T. Liu et al. appearing in the Journal of Atmospheric Science, 36, 1722-1735.